The present invention relates generally to the boring of horizontal holes and, more particularly, to the guiding of a horizontal boring tool. Still more particularly, the present invention relates to the control of the guidance of such a tool, especially using a magnetic sensing system for sensing tool location and attitude.
The present invention has particular application in the installation of conduits and pipes by various utilities, such as gas, telephone and electric utilities. Such utilities are often faced with the need to install or replace such conduits or pipes under driveways, roads, streets, ditches and/or other structures. To avoid unnecessary excavation and repair of structures, the utilities use horizontal boring tools to form the bore holes in which to install the conduits or pipes. Such tools have been unsatisfactory to the extent that their traverse has not been accurate or controllable. All too frequently other underground utilities have been pierced or the objective target has been missed by a substantial margin. It has also been difficult to steer around obstacles and get back on course.
The directional drilling of holes has probably reached its greatest sophistication in the oil fields. Typical well surveying equipment utilizes magnetometers, inclinometers and inertial guidance systems which are complex and expensive. The wells drilled are substantially vertical. In respect to utilities, Bell Telephone Laboratories Incorporated has designed a system for boring horizontal holes wherein the direction of drilling is controlled by deploying a three wire antenna system on the surface of the earth and detecting the position and attitude of the drilling tool in respect thereto by pickup coils on the tool. The signals detected are then used to develop control signals for controlling the steering of the tool. See, for example, MacPherson U.S. Pat. No. 3,656,161. Such control systems have been relatively expensive, and it is not always easy or convenient to deploy the antenna, for example, over a busy highway.
Steering control is also known in controlling vehicles, aircraft and missiles. In one form of control, a radio beacon is used for guidance, the aircraft simply following a beacon to a runway.